1. Field of the Invention
The invention relates generally to photonic applications. More specifically, the invention relates to a method for forming a planar waveguide structure.
2. Background Art
Planar waveguide structures, such as employed in planar amplifiers and planar waveguide lasers, are desirable in micro-optics because they are compact (in comparison to fiber-based waveguide structures) and can be integrated on the same chip with other components. Generally speaking, planar waveguide structures, include a planar core layer supported on a substrate and a cladding layer formed on the core layer. The substrate and cladding have a lower refractive index than the core layer so that optical radiation is confined by total internal reflection within the walls separating the core layer from the substrate and cladding. Typically, the core layer comprises an array of waveguide cores (or dielectric strips) within which optical radiation propagates.
Current methods for making planar waveguide structures such as described above involve providing a substrate having a clean flat and smooth surface. Typically, the substrate is made of silicon or silica. A material having a high refractive index, typically a silicate, is then deposited on the substrate to form the core layer. For applications such as planar amplifiers, the core layer is doped with an optically active element, typically a rare-earth metal such as erbium. Such an optically active element is excited by laser light at a selected wavelength to produce more light (amplification) at the same wavelength. The core layer is patterned to form a waveguide pattern, usually by some variation of a lithography/etching process or bias-sputtering/etching process. After forming the waveguide pattern, a low-index cladding layer, e.g., silica, is deposited on the waveguide pattern to form the complete waveguide structure. If the substrate is made of a high-index material such as silicon, a low-index buffer layer is typically deposited on the substrate prior to depositing the core layer.
The two main materials currently used for fabricating planar waveguide structures are crystalline materials, such as LiNbO3, Al2O3, and Y2O3, and glass materials, such as silica-based glass and phosphate-based glass. Recently, there has been an interest in using transparent glass-ceramics in photonic applications. The interest arises from the desirable optical properties of transparent glass-ceramics doped crystals for such photonic devices as lasers and amplifiers. Transparent glass-ceramics also offer the advantage of glass fabrication together with the optical behavior of a crystal. As an example, U.S. application Ser. No. 09/686,564 (the '564 application) by Beall et al, supra, discloses a transparent glass-ceramic that provides gain over every wavelength that is conceivably of interest in telecommunications today. The glass-ceramic gain media comprises a transition-metal-doped glass in which extremely small crystals are internally nucleated. The crystals are formed from constituent materials of the original glass melt and are uniformly dispersed throughout the glass. Because the gain media is glass-based, it can be readily spliced to silica glass fibers. The '564 application discloses Cr+4/forsterite glass-ceramic materials which emit at wavelengths ranging from about 900 nm to 1400 nm and Cr+4/willemite glass-ceramic materials which emit at wavelengths ranging from about 1100 nm to about 1700 nm.
The crystals in the glass-ceramic provide the glass-ceramic with a bulk refractive index different from the precursor glass material. Waveguide structures can take advantage of this feature if the crystals can be locally produced along a given track. The '564 application describes an internal nucleation method by which crystals are uniformly dispersed through the glass material. To produce a waveguide pattern, better control is needed over where the crystals are formed in the glass material. Therefore, what is desired is a method of locally producing a crystalline phase in a glass material. U.S. application Ser. No. 09/607,631 (the '631 patent) by Borrelli et al, supra, discloses a method for patterning an optical material on an optical element. In embodiments disclosed in the '631 patent, an energy source, such as a CO2 laser, was used to locally heat a birefringent glass having ellipsoidal metal halide particles dispersed therein. The localized heating resulted in the ellipsoidal metal halide particles relaxing to form spheres and removed the birefringence from the locally heated regions.